The instant invention relates to electrical cables and more particularly to an improved shielded electrical cable construction of a type which includes a shielding layer comprising a flexible metallic shielding tape.
The concept of shielding various electrical components against the ingress and egress of stray electrical currents or radiation is well known in the electronics industry. In this connection, shielded electrical cables comprising a metallic shielding layer for shielding against the ingress and egress of radiation are widely used for transmitting electrical signals in a broad range of applications. Cables of this type generally comprise an inner conductor wire, a dielectric core over the conductor wire, a shielding layer over the dielectric core, and an insulating outer jacket over the dielectric core. The shielding layers of most conventional shielded cables of this type generally comprise either a flexible layer of thin braided wire strands and/or a thin flexible metallic shielding tape, for example, a tape made of plastic film-foil laminate. Generally, it has been found that shielded cables including shielding layers of these types can be utilized for transmitting electrical signals so that they are less affected by electromagnetic radiation emanating from adjacent cables or other components and also so that they produce reduced quantities of radiation which can affect the signals transmitted through other cables or components.
However, while cables comprising shielding layers made from a plurality of thin braided wire strands are generally satisfactory for some applications, it has been found that the unavoidable inherent openings in braided shielding layers of cables of this type can be penetrated by radiation, particularly radiation which is produced by high frequency signals, for example, those signals which are often required for high speed communication of computer data. Further, it has been found that cables of this type are relatively susceptible to moisture penetration when they are utilized in exterior applications. Specifically, it has been found that once moisture has penetrated the outer jacket of a cable of this type, it can easily penetrate the braided shielding layer of the cable. Further, it has been found that once moisture has penetrated the shielding layer of a cable of this type, it has a tendency to be carried along the length of the cable via capillary action or wicking effects until the water eventually reaches a defect in the core where it can penetrate through to the conductor wire of the cable. Still further, it has generally been found that once moisture has penetrated through to the core of a cable in this manner, it can cause the cable to fail due to corrosion and/or short circuiting.
On the other hand, cables comprising shielding layers made from metallic shielding tapes have generally been found to be substantially less prone to passing stray electrical currents or radiation through the shielding layers thereof than cables comprising shielding layers made from braided wire strands. Generally, the shielding layers of cables of this type are formed by either spirally or longitudinally "cigarette" wrapping metallic foil tapes on the outer surfaces of the dielectric cores thereof. In many instances, the metallic foil shielding tapes utilized in cables of this type include waterproof adhesive layers on the inner surfaces thereof for adhesively securing them to the respective dielectric cores thereof. Further, generally when metallic foil tapes of this type are assembled on the dielectric cores of cables to form shielding layers thereon, they are assembled so that the opposite edge portions of the tapes overlap one another to form overlapping seams in order to minimize radiation leakage through the seam areas. However, it has also been found that when tapes having adhesive layers on the inner surfaces thereof are assembled in cables in this manner, the adhesive layers normally prevent metal-to-metal contact in the overlapping seam areas. Further, it has been found that the thicknesses of the adhesive layers of shielding tapes of this type can cause the metallic layers of such tapes to be spaced apart slightly in the overlapping seam areas thereof, and that although the spacings between the metallic layers in such seam areas are relatively small, they are nevertheless suffficient to provide paths which permit the ingress and egress of certain quantities of electromagnetic radiation. Nevertheless, it has generally been found that the use of adhesives to secure the edge portions of metallic foil tapes together in the overlapping seam areas of shielding layers is generally essential to prevent moisture penetration through shielding layers.
The instant invention provides a novel solution to the problem of minimizing the leakage of radiation through the overlapping seams areas of the shielding layers of cables comprising metallic foil shielding tapes. Specifically, the instant invention provides a shielded cable construction comprising an elongated conductor wire, a dielectric core over the conductor wire, and a metallic shielding tape over the dielectric core. The shielding tape includes opposite first and second edge portions, and it is wrapped over the dielectric core so that the second edge portion overlaps the first edge portion. Further, the shielding tape includes a metallic layer having inner and outer surfaces and an adhesive layer on preselected portions of the inner surface of the metallic layer in at least the second edge portion of the tape to define a plurality of slightly recessed, localized contact pads on the second edge portion of the tape wherein the metallic layer is exposed. The outer surface of the metallic layer is exposed in at least the first edge portion of the tape, and the inner surface of the second edge portion of the tape is bonded to the outer surface of the first edge portion so that the inner surface of the metallic layer contacts the outer surface of the metallic layer in at least a portion of the contact pads. The adhesive layer of the tape is preferably formed so that it provides a substantially continuous seal between the first and second edge portions of the tape in the overlapping seam area which extends along the longitudinal extent of the cable despite the presence of the contact pads in the seam area. Further, the contact pads are preferably formed so that they are of substantially uniform width and they are preferably oriented so that they are aligned in at least one substantially longitudinally extending row of contact pads which extends along the second edge portion of the tape. Further, the width of the overlapping portion of the tape is preferably greater than the width of the contact pads in the seam area so that an effective seal can be provided between the inner and outer surfaces of the tape in the overlapping seam area while nevertheless providing metal-to-metal contact between the inner and outer surfaces of the metallic layer in the contact pads. The contact pads on the metallic layer are preferably formed so that they have longitudinal and transverse dimensions which are at least five times the thickness of the adhesive in order to further assure that effective metal-to-metal contact is achieved between the inner and outer surfaces of the metallic layer in the contact pads. Further, the adhesive layer and the contact pads are preferably formed so that metal-to-metal contact between the inner and outer surfaces of the metallic layer is achieved along at least 10% of the length of the cable. A first embodiment of the shielded cable includes a single shielding layer comprising a metallic foil shielding tape, and a second embodiment of the cable includes a second braided wire shielding layer over the metallic foil shielding tape thereof. The shielded cable preferably further comprises an outer insulating jacket either directly over the metallic shielding tape or over the braided wire shielding layer, and the adhesive layer preferably extends over preselected portions of the entire inner surface of the metallic layer to bond and seal the metallic tape to the dielectric core. The metallic layer in the shielding tape can be effectively embodied as comprising a single metal sheet or a plurality of metal sheets with at least one plastic reinforcing film interposed therebetween.
Cables and shielding materials representing the closest prior art to the subject invention of which the applicant is aware are disclosed in the U.S. Pat. Nos. to CLUNE, #1,445,070; TIMMONS, #3,274,329; MYERS, #3,413,405; OLYPHANT, JR. et al, #3,497,383; KILDUFF et al, #3,505,144; ANGELE, #3,612,743; GREY, #3,662,090; SWEARINGEN et al, #3,770,570; STINE et al, #4,096,346; VERMA et al, #4,209,215 and GRUHN, #4,596,897 and the European patent to OLYPHANT JR., #0 061 829. However, these references fail to suggest the concept of providing an interrupted adhesive layer on portions of a metallic shielding tape in order to achieve metal-to-metal contact in the overlapping seam in a shielding layer while nevertheless providing an effective water-tight seal in the overlapping seam area. As a result, the above references are believed to be of only general interest with respect to the shielded cable of the subject invention.
Accordinlgy, it is a primary object of the instant invention to provide an improved shielded cable construction of a type which includes a shielding layer comprising a metallic foil tape.
Another object of the instant invention is to provide a shielded cable construction of a type which includes a shielding layer comprising a metallic shielding tape wherein overlapping portions of the shielding tape are adhesively secured together to form an overlapping seal and wherein metal-to-metal contact is nevertheless effected in the overlapping seal.
A still further object of the instant invention is to provide a shielded cable construction comprising a conductor wire, a dielectric core over the conductor wire, and a metallic foil shielding tape over the dielectric core wherein the shielding tape includes an adhesive layer on preselected portions thereof, wherein one edge portion of the shielding tape is adhesively secured in overlapping relation to the opposite edge portion thereof with the adhesive layer, and wherein a plurality of open contact patches are provided in the adhesive layer to effect metal-to-metal contact between the overlapping opposite edge portions of the tape.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.